As the Mallozzi and Fairand patent points out, old methods for the shock processing of solid materials typically involve the use of high explosive materials in contact with the solid, or high explosive materials are used to accelerate a plate that strikes the solid to produce shock waves therein. Such methods have several disadvantages. For example: (a) it is difficult and costly to shock process non-planar surfaces and complicated geometries, (b) storage and handling of the high explosive materials pose a hazard, (c) the processes are difficult to automate and thus fail to meet some industrial needs, and (d) the high explosive materials cannot be used in extreme environments such as high temperatures and high vacuum.
Shot peening is another widely known and accepted process for improving the fatigue, hardness, and corrosion resistance properties of materials by impact treatment of their surfaces. In shot peening, many small shot or beads are thrown at high speed against the surface of a material. The shot or beads sometimes escape from the treatment equipment and scatter in the surrounding area. Since particles might get into surrounding machinery and cause damage, shot peening usually cannot be used in a manufacturing line. Ordinarily it cannot be used on machined surfaces without damaging them.
Laser shock processing equipment, however, can fit right into manufacturing lines without danger to surrounding equipment. It is also readily adaptable to automatic control, making it further attractive for production line applications. It can be used on machined surfaces of harder metals and alloys with no damage to the surfaces.
The interaction of a pulsed laser beam with the surface of a material gives rise to a pressure pulse (shock wave) that propagates into the material and changes its properties. In the case of metals, for example, the changes in properties are caused by the introduction of cold work that increases the hardness and strength of the material. By appropriate tailoring of the peak pressure and width of the shock wave, it is possible to enhance selected material properties, such as fatigue strength, and at the same time not adversely affect other properties, such as corrosion resistance. It is possible also to shock process a finished piece of material without disturbing its surface, where a thin sacrificial layer of overlay material has been attached intimately onto the surface of the workpiece.
Shock processing with coherent radiation has several advantages over what has been done before. For example: (a) The source of the radiation is highly controllable and reproducible. (b) The radiation is easily focused on preselected surface areas and the operating mode is easily changed. This allows flexibility in the desired shocking pressure and careful control over the workpiece area to be shocked. (c) Workpieces immersed in hostile environments such as high temperature and high vacuum can be shock processed. (d) It is easy to shock the workpiece repetitively. This is desirable where it is possible to enhance material properties in a stepwise fashion. Shocking the workpiece several times at low pressures can avoid gross deformation and spallation of the workpiece. (e) The process is readily amenable to automation. (f) Nonplanar workpieces can be shock processed without the need of elaborate and costly shock focusing schemes.
As mentioned in the patent of Mallozzi and Fairand, several publications have dealt with the use of lasers to provide stress waves in solids:
1. G. A. Askar'yan and E. M. Moroz, JETP Letters 16. 1638 (1963). PA0 2. Frank Neuman, Appl. Phys. Letters 4, 167 (1964). PA0 3. David W. Gregg and Scott J. Thomas. J. Appl. Phys. 37, 2787 (1966). PA0 4. C. H. Skeen and C. M. York, Appl. Phys. Letters 12, 369 (1968). PA0 5. N. C. Anderholm, Appl. Phys. Letters 16, 113 (1970). PA0 6. S. A. Metz and F. A. Smidt, Jr., Appl. Phys. Letters 19, 207 (1971). PA0 7. L. C. Yang and Vincent J. Menichelli, Appl. Phys. Letters 19, 473 (1971). PA0 Soviet Physics-Doklady, Vol. 14, No. 11, May 1970, pgs. 1128-1130. PA0 Soviet Physics-Technical Physics, Vol. 12, No. 6. Dec. 1967, pgs. 753-757. PA0 A. B. P. Fairand, B. A. Wilcox, W. J. Gallagher, and D. N. Williams, J. Appl. Phys., Vol. 43, pp. 3893-3895 (1972). PA0 B. A. H. Clauer, B. P. Fairand, and B. A. Wilcox, Met. Trans. A., Vol. 8A, pp. 119-125 (1977). PA0 C. A. H. Clauer, B. P. Fairand, and B. A. Wilcox, Met. Trans. A., Vol. 8A, pp. 1871-1876 (1977). PA0 D. B. P. Fairand and A. H. Clauer, J. Appl. Phys., Vol. 50, pp. 1497-1502 (1979). PA0 E. A. H. Clauer and B. P. Fairand, Applications of Lasers in Materials Processing, ed. by E. Metzbower, American Society for Metals, Metals Park, Ohio (1979). PA0 F. A. H. Clauer, J. H. Holbrook, and B. P. Fairand, Shock Waves and High Strain Rate Phenomena in Metals, ed. by M. A. Meyers ard L. E. Murr, Plenum Press, New York (1981), pp. 675-702. PA0 G. S. C. Ford, B. P. Fairand, A. H. Clauer, and R. D. Galliher, Investigation of Laser Shock Processing, Final Report, AFWAL-TR-80-3001, Vol. II (August, 1980). PA0 H. A. H. Clauer, C. T. Walters, and S. C. Ford, The Effects of Laser Shock Processing on the Fatigue Properties of 2024-T3 Aluminum, Lasers in Materials Processing, Ed. by E. A. Metzbower, American Society for Metals, Metals Park, Ohio, 1983, pp. 7-22. PA0 I. Ichiyama at al, U.S. Pat. No. 4,293,350, Oct. 6, 1981; Grain-oriented Electromagnetic Steel Sheet with Improved Watt Loss. PA0 J. T. Iuchi, S. Yamaguchi, and T. Ichiyama, Laser processing for reducing core loss of grain oriented silicon steel, J. Appl Phys. 53(3), March 1982, pp. 2410-2412. PA0 K. Jean Fournier, Remy Fabbro, J. L. Strudel, and D. Ayrault, Experimental Study of Deformation Induced in Metallic Alloy Laser Generated High Pressure Shocks, Conference: LAMP '87: Laser Advanced Materials Processing----Science and Applications, Osaka, Japan, 21-23 May 1987. High Temperature Society of Japan, c/o Welding Research Institute of Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567, Japan, 1987, pp. 365-370. PA0 L. R. Fabbro, J. Fournier, E. Fabre, E. Leberichel, Th. Hannau, C. Corbet, Experimental study of metallurgical evolutions in metallic alloys induced by laser generated high pressure shocks, Proceedings of SPIE - The International Society for Optical Engineering v 668. Publ. by SPIE, Bellingham, Wash., USA, 1986, pp. 320-324.
The majority of these older papers were concerned with the phenomenology of lasergenerated pressure pulses. Exceptions include an experiment where pressure pulses generated in very thin aluminum foils were used to detonate insensitive high explosives, and a study of vacancy production in thin vanadium and nickel foils, as reported in the last two papers listed above. The early studies did not look at the possible use of pulsed lasers to significantly alter in-depth material properties such as dislocation substructures. Because of its high controllability and reproducibility, a pulsed laser provides an important tool for studies of basic mechanisms of shock deformation of solids, as well as practical material shock processing applications.
The papers listed as numbers 6 and 7 above were cited in the examination of the patent of Mallozzi and Fairand. Also cited were the following United States Patents and other publications, which were considered to be of interest to show shock hardening of metals, producing vacancy sites in metals by laser bombardment, and the effects of laser bombardment on metals.
______________________________________ UNITED STATES PATENTS ______________________________________ 2,703,297 3/1955 MacLeod 148/4 3,172,199 3/1965 Schmidt 148/4 X 3,218,199 11/1965 Cowan et al 148/4 3,454,435 7/1969 Jacobs 148/12.7 ______________________________________